Magnetically attractable toner powders have found widespread use for generating visible information on an information support. They are used, inter alia, not only in magnetographic printing or reproduction processes but also in electrophotographic and electrostatic processes in which the magnetic properties of the toner powder are utilized to enable the powder to be fed by means of magnetic conveying means to a developing or image-forming station. The readily magnetizable materials conventional in toner powders, such as carbonyl iron, ferrites and chromium dioxide, are brown to brown-black in color and fulfill the function of coloring material in black toner powders, usually in addition to carbon.
In the production of colored magnetically attractable toner powders, the presence of the dark tinted magnetically attractable material also gives rise to problems because its dark tint should be masked to enable colored toner powder to be obtained with a high brightness and good color saturation.
Japanese Patent Application No. 76/46131 proposes the production of colored toner powder by enveloping the magnetically attractable particles chemically with a white substance or dispersing the magnetically attractable material together with white pigment in a binder, grinding the mass into particles and finally coating these particles with a colored polymer layer.
A similar proposal is made in German Patent Application No. 3542834, in which colored toner powder is made by dispersing round or elliptical magnetically attractable particles and color pigment in a thermoplastic binder and grinding the mass into particles of the required particle size.
Colored toner powders having attractive bright and saturated colors are not obtained with the above proposals and this applies particularly when yellow or red toner powders are required. See also Japanese Patent Application Nos. 58/14842 which discloses toner powder comprising yellow fluorescent pigment or dye and 57/66441 which discloses using a fluorescent material having a wavelength peak less than or equal to 420 nm.
European Patent Application Nos. 75346 and 156408 describe colored magnetically attractable toner powder which consists of a magnetically attractable core, a masking layer which envelops the core and which contains binder and light-reflecting pigment, and coloring constituents which are disposed in and/or on the masking layer. The color characteristics of these layered toner powders are considerably better than those of the previously-mentioned toner powders. The objection is again that the manufacture of such toner powders is complicated. Bright saturated colors can also be obtained only if a fairly thick masking layer and a relatively thick coloring layer are applied successively. The treatments then result in relatively coarse toner particles which are unsuitable for reproducing high-resolution patterns. Yet another disadvantage of these toner powders is fixing by means of pressure and heat involves the risk of the toner particles being excessively spread out, so that the dark core is exposed more or less and consequently the color quality of the fixed images is clearly poorer than that of the toner powder itself. This disadvantage may occur particularly in the production of double-sided prints, in which the first side of a receiving support is first provided with a fixed image and then the other side. The first image formed is then subject to double fixing and as a result gives a distinctly poorer color quality than the last image formed.
It is, therefore, an object of the present invention to provide a colored magnetically attractable toner powder without the above disadvantages or in which the above disadvantages are significantly reduced.